The present invention relates to ball valves of the top entry type, and is more particularly concerned with a top entry ball valve that is adapted to receive a conventional valve ball and associated seat rings of conventional design through an opening in the valve housing without regard to the orientation of the ball.
Most ball valve designs feature body joints which are subject to piping stress, and this can cause leakage. If such joints are welded, the valve is usually rendered unserviceable, and cannot be repaired, and the welding process itself may cause damage to the seat and seals within the valve housing. Top entry valves avoid these particular problems since they have a bonnet joint that is not subject to piping stress, and top entry valves can be disassembled for weld-in installation thereby protecting the seats and seals from damage due to the heat of welding.
Top entry ball valves of current design have assembly problems, however. The seats should be preloaded against the ball to provide true sealing capability. The required preload causes the assembly problem. Various arrangements have been suggested to facilitate assembly of top entry valves, each of which, however, gives rise to still other problems.
In one approach suggested heretofore the valve employs tapered arrangements to wedge the seats against the ball. Such valves, when used under varying service conditions, e.g., high temperature followed by low temperature, with repeated thermal cycling, are subject to "thermal jacking" caused by differential expansion, i.e., every time there is a significant reduction in the temperature of the valve, the ball/seat package shrinks and falls further into the valve body. When the valve is subsequently heated again, and the ball/seat package expands, the valve operating torque can become very high.
An alternative approach has been to modify the configuration of the ball that is used in the top entry valve so that the ball is foreshortened across the port hole in the ball, i.e., a portion of the outer diameter of the ball is flattened. When the ball is placed in open position between a pair of annular seat rings, the "stack height" of the ball/seat package is less than the height of the package with the ball in its closed position, the difference in these stack heights being sufficient to allow the open ball stack to be inserted into the valve body. Designs of this type, however, require flexible seats, thereby limiting the service conditions under which the ball valve can be used, and each cycle flexes the seats causing additional wear on all seat sealing surfaces. Moreover, the ball is loose when it is in its open position, which is undesirable, and particles of dirty media can be trapped behind the seats, causing leakage and failure.
In an effort to avoid some of the foregoing problems, it has been suggested that the valve be provided with a mechanism of some type that can be used to axially load the seats onto the ball after the ball/seat package has been inserted into the valve body, for example by adjusting the axial positions of the seats relative to one another and relative to the valve ball. Externally adjustable designs have been suggested employing, for example, adjustment mechanisms that penetrate the body wall and are manipulated from the exterior of the valve body, but such penetrations are undesirable since they provide potential leakage paths. Internally adjustable designs have been suggested which feature jacking screws or similar devices, but devices of this type are subject to corrosion, stress cracking, or gumming and sedimentation (deposits) which impede operation of the device used to axially load the seats.
Spring loaded designs have also been suggested, but springs under stress are much more prone to corrosion and failure than similar material used in an unstressed condition in a similar environment. Stress corrosion is a very common failing, and, in addition, the loading provided by springs may vary significantly under particular service conditions since, for example, springs relax at high temperatures.
The present invention is intended to obviate the foregoing problem.